The invention relates to a switching device, and more particularly, to a switching device having a memory function, wherein the memory/switching device can be addressed and the switching state reversed by externally applying an electric field.
Various types of electrically addressable memory devices for computer data storage are known in the art. Most of these devices store a data bit as a charge in a capacitor. The charge state can be read out and the output signal used to control processes in a computer processor. Most of these devices require complex silicon processing steps and a dedicated device architecture which depends on memory type.
Memory devices are distinguished by their speed and data retention characteristic. Dynamic random access memory (DRAM) is a volatile memory characterized by a destructive read. This means that it is necessary to supply voltage to the memory bits at all times, or the information will disappear. Furthermore, each memory element has associated with it a transistor. Static random access memory (SRAM) stores data in a bistable flip-flop, commonly consisting of cross-coupled inverters. It is called xe2x80x9cstaticxe2x80x9d because it will retain a value as long as power is supplied. It is still volatile, i.e. it will lose its contents when the power is switched off, in contrast to ROM. SRAM is usually faster than DRAM, but each bit requires several transistors (about six), so that a lesser number of bits of SRAM fit in the same area as compared to DRAM.
Erasable programmable read only memory (EPROM) is a type of storage device in which the data is determined by electrical charge stored in an isolated (xe2x80x9cfloatingxe2x80x9d) MOS transistor gate. The isolation is good enough to retain the charge almost indefinitely (more than ten years) without an external power supply. The EPROM is programmed by xe2x80x9cinjectingxe2x80x9d charge into the floating gate, using a technique based on the tunnel effect. This requires higher voltage than in normal operation (usually 12V-25V). The floating gate can be discharged through UV-illumination or electrically (EEPROM). Usually bytes or words can be erased and reprogrammed individually during system operation. EEPROM is more expensive and less dense than RAM. It is appropriate for storing small amounts of data which is changed infrequently. Another known non-volatile memory device is a ferromagnetic RAM (Fe-RAM), wherein the individual storage cells do not require a dedicated transistor.
It would therefore be desirable to provide a device for data storage and data manipulation that is easy and inexpensive to manufacture and has a predictable memory and/or switching effect.
The present invention provides a novel switching device that comprises first and second electrodes, an active region provided between the first and second electrodes and including a molecular system, and a control electrode for controlling an electric field applied to the active region. Ionic complexes may be distributed in the molecular system.
In accordance with an embodiment of the present invention, the electric field is controlled for switching the active region between a high-impedance state or off state and a low-impedance state or on state. The reproducible switching between the off state and the on state may be achieved by applying an electrical field with a predetermined polarity and magnitude for a predetermined time.
In particular, the active region may be switched from the off state into the on state when a voltage of a first polarity is applied between the control electrode and the first and second electrodes. As a result, an electrically conducting channel is provided between the first and second electrodes. The active region may be switched back to the off state when a voltage of a second polarity opposite with respect to the first polarity is applied between the control electrode and the first and second electrodes.
The ionic complexes dissociate under the influence of an external applied electric field, thereby changing the electrical conductivity of the device. The molecular matrix can be polyconjugated compounds, aromatic and heterocyclic molecules, quasi-one-dimensional complexes, such as phthalocyanines and porphyrins, and anisotropic inorganic materials, such as NbSe3. Each of the ionic complexes may include, for example, ions of Na and Cl or ions of Cs and Cl.
The active region in accordance with an embodiment of the present invention may provide stable and metastable modes of operation. Intensity of an electric field required to switch the switching device between the on and off states in the stable mode is higher than in the metastable mode. The on-state impedance of the active region in the stable mode is lower than in the metastable mode, whereas the switching time as well as data storage time in the stable mode is longer than in the metastable mode.
In accordance with an embodiment of the invention, the first and second electrodes of the switching device may be provided on an insulating substrate. The active region also may be provided on the substrate so as to have an electrical contact with the first and second electrodes. The control electrode may be formed on an insulating layer covering the active region.
In accordance with another embodiment of the invention, no insulating substrate is required. The active region may be sandwiched between the first electrode and the second electrode. The control electrode may be provided on the insulating layer covering the first electrode.
In accordance with another aspect of the invention, a memory switch is provided with a gate terminal, at least two signal terminals, and an active element including a molecular system and ionic complexes distributed in the system. The active element electrically connected between the signal terminals is responsive to a signal applied to the gate terminal for switching between an on state and an off state.
The foregoing and other features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.